A first cut was successfully made in the leaking pipe overnight, and Allen said he expected the saw would be extracted, or that a new saw would be sent down to the wrecked well head later today.

Once the riser pipe has been cut off, a "cap" will be lowered over the top of the well head to contain the oil, which will be siphoned up to a drill ship on the surface through a riser extending from the top of the containment unit.

Every attempt so far to plug the leak has failed, and BP has stressed this latest attempt will not stop the flow of oil but only stem it until two relief wells have been drilled. The drill wells are expected to be finished in August.

At least 175 million litres of oil are estimated to have gushed into the Gulf since the April 20 explosion that sent the Deepwater Horizon rig sinking to the seabed 90km off the Louisiana coast.